The present invention relates to an apparatus and method for automatically placing an array of solder balls onto a substrate, such as a ball-grid array (BGA) substrate.
The use of BGA packages as semiconductor integrated circuits is becoming more popular and common. One method of manufacturing BGA packages is to firmly attach solder balls to solder pads (or electrodes) found on a BGA substrate such that electrical conductivity is maintained between electrical connections manufactured on the BGA substrate and external devices with corresponding electronic circuits. Such a manufacturing process typically involves solder ball placement and solder ball reflow. The former involves locating the positions of the solder pads and placing the solder balls onto these positions, and the latter is a process in which solder balls melt, wet and reflow onto the solder pads of the BGA substrate to create the electrical conductivity.
Current systems for solder ball placement usually apply xe2x80x9csweepingxe2x80x9d and xe2x80x9cgravity rollingxe2x80x9d concepts to locate and place the solder balls onto the solder pads. Several examples from the prior art will illustrate the concept of sweeping.
U.S. Pat. No. 5,655,704 for a xe2x80x9cMethod and Apparatus for Mounting Soldering Balls onto Electrodes of a Substrate or a Comparable Electronic Componentxe2x80x9d describes a ball-filling device using a xe2x80x9csweepingxe2x80x9d concept. The ball-filling device include a container which has a supply of solder balls, which fill up apertures of a ball-receiving element when the container is moved over the area of the apertures. Due to the storage of a large number of solder balls in the container, significant forces are generated due to the weight of the plurality of solder balls which are exerted on one another, and these forces act to restrain the solder balls from falling freely under the influence of gravity into the apertures. In addition, there is a high risk that solder balls that are partially engaged in empty apertures are sheared by the edges of the container as the container is moved over the apertures.
Another example of a ball-filling device is described in PCT publication number 00/564921 for a xe2x80x9cFilling Device and Method for Filling Balls in the Apertures of a Ball-Receiving Elementxe2x80x9d. It comprises housing including an opening at a lower surface thereof and defining in part a chamber for containing a supply of balls. The housing is movably disposed over an array of apertures, and distribution means disposed within the housing distributes balls contained in the chamber so that limited number of layers of the balls are spread over the apertures. Although the described device alleviates the problem of shearing of solder balls during filling of the solder pads as compared to the prior art, the soft solder balls can still be damaged by the distribution means, which in itself, has edges and produces undesired forces on the solder balls that can cause damage.
Another example of the use of these concepts is U.S. Pat. No. 6,229,210 which describes a xe2x80x9cDevice and method for attaching and soldering pre-formed solder spheres to the BGA integrated circuit package attachment sites in high volume.xe2x80x9d In this design, the placement device comprises a pre-formed stencil having a pattern of through holes corresponding to contact pads of a BGA substrate. By brushing the solder balls or by the use of a U-shaped blade, the balls are made to fall into the apertures. The problem is that since the solder balls are exposed to the atmosphere, they can be oxidized more easily and contamination may easily occur from the surroundings and/or the blade. Furthermore, the soft solder balls may be damaged by the blade.
As another example, U.S. Pat. No. 5,918,792 describes an xe2x80x9cApparatus and method for filling a ball grid arrayxe2x80x9d. A ball-filling device comprises a solder bin that is moved along side rails over a stencil allowing solder balls to fill apertures in the stencil and thereby settling on corresponding solder pads, before being fixed by adhesive. As explained above, the problem is that solder balls may be damaged by the edges of the side walls of the solder bin and also by the edges of the many apertures.
It Is an objective of the present Invention to provide an improved apparatus for automated placement of solder balls onto a substrate, so that damage and/or oxidation of solder balls are reduced as compared to the prior art.
According to a first aspect of the invention there is provided an apparatus for positioning solder balls in a desired array on a substrate comprising a positioning means for positioning the solder balls in positions corresponding to the array of positions the solder balls are to take up on the substrate, a container for receiving a plurality of solder balls and which is movable between a first position remote from the positioning means and a second position directly thereover, and means to bias the solder balls in the direction of movement of the container from the first to the second position whereby to reduce or obviate damage to the solder balls during such movement.
According to a second aspect of the invention there is provided a method for positioning solder balls in a desired array on a substrate, comprising providing positioning means for positioning the solder balls in positions corresponding to the array of positions the solder balls are to take up on a substrate, providing a container for receiving a plurality of solder balls, providing means to move the container between a first position remote from the positioning means and a second position directly thereover, and providing means to bias the solder balls to move in the direction of movement of the container whereby damage to the solder balls is reduced or obviated.
According to a third aspect of the invention there is provided a substrate on which a plurality of solder balls so arranged in a desired array according to an apparatus or a method as hereinbefore defined.
Using the invention it is possible to provide an apparatus and method in which a force, usually gravitational force, will be provided which is independent of the force generated on the solder balls by the walls of the container during said movement whereby to bias the solder balls In the direction of movement of the container.
It will be convenient to hereafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.